Coating opposite sides of web employing electrostatic field



April 1968 u. G. SHAPIRO ETAL 3,380,845

COATING OPPOSITE SIDES OF WEB EMPLOYING ELECTROSTATIC FIELD 2 Sheets-Sheet 1 Filed Dec. 6, 1965 A TTORNE);

p 0, 1968 U.G.SHAPIRO ETAL 3,380,845

COATING OPPOSITE SIDES OF WEB EMPLOYING ELECTROSTATIC FIELD Filed Dec. 6, 1963 2 Sheets-Sheet 2 SUPPLY PIPE l9 SPRAYING HEAD 6. 2

INVENTORS URIEL G. SHAPIRO FRANCIS G. NEARY BY WJpMJW W ATTORNEYS United States Patent 3,380,845 COATING OPPOSITE SIDES 0F WEB EMPLOYING ELECTROSTATIC FIELD Uriel George Shapiro, London, and Francis Charles Neary, Newcastle-upon-Tyne, England, assignors to Commercial Plastics Limited, Wallsend, Northumberland, England Filed Dec. 6, 1963, Ser. No. 328,645 11 Claims. (Cl. 117-68) The present invention relates to the production of coated webs of electrically non-conducting sheet material and provides a method of and apparatus for coating such webs.

Webs of sheet material such as paper or film are commonly coated with materials such as synthetic resins to modify their properties, for example to reduce their permeability to air, moisture or other media, and coatings of such materials may be applied to both sides of webs, for example to protect the material of the web or to provide scalable surfaces on both faces of the web.

It has been found that very even layers of coating materials can be obtained by the methods of electrostatic spraying, the sprayed material being deposited under the influence of an electrostatic field after being atomised either mechanically or by the action of the field itself. Where a web is non-conducting an earthed counter-electrode, for example a metal roller, is placed immediately behind the web.

It has now surprisingly been found that if a non-conducting web, which cannot of course be earthed, is passed between spray heads between which a high tension electrostatic field is maintained, a coating is formed on each side of the web.

Accordingly, the present invention provides a method of coating both sides of a web of electrically non-conducting sheet material which consists in passing the web between spray heads between which a high tension electrostatic field is maintained and causing coating material to be atomised at the heads and to be deposited on the web by the action of the field.

The individual spray heads may be charged to any desired potentials which result in the establishment of a high tension field, but it is convenient for the heads on opposite sides of the web to be charged to potentials of substantially equal magnitude but opposite sign.

Similarly, the invention provides apparatus which includes two spaced spray heads, means for supplying coating material to the spray heads, web advancing means for drawing the web between the heads and means for maintaining a high potential difference between the heads.

In the preferred method and apparatus, the heads are spaced horizontally from one another and the web is drawn between them in a generally vertical plane. The plane of the web is preferably equidistant from the two heads. By these means similar coatings are formed on the two sides and any differential effect due to gravity is avoided.

Although spray heads relying on mechanical atomisation, for example air-operated atomisers, can be used, the currently preferred heads rely on electrostatic atomisation. One form of head, which yields coatings of liquid coating compositions of exceptional evenness and can readily be operated to provide a horizontal spray, comprises an inclined plane of which the free lower edge is formed as a sharp edge for electrostatic atomisation of the coating composition, which is caused to flow evenly down the plane. Spray heads of this form are described and claimed in our co-pending application No. 295,995.

Double coatings may be produced according to the invention for the purpose of protecting the base sheet material, for example coating hygroscopic regenerated cellulose sheet to render it moisture proof. Another application of the method is in the production of scalable sheets of films, for example plastic-coated cellulose or polypropylene film. Moreover, the employment of the invention also results in the application to the web of a greater weight of coating material per unit area, so increasing the degree of modification of any property which depends on the thickness of coating applied.

FIG. I is a perspective view of the coating apparatus of the present invention; and

FIG. 2 is a view of the supply pipe and trough from which the material is supplied to the spray heads.

Let-01f and take-up devices of conventional form not shown in the drawing are arranged to draw a web of paper or synthetic resin film, for example of polypropylene, indicated in broken lines at 10, over a pair of vertically spaced parallel guide rollers, of which only the lower roller 11 is shown in the drawing. Spray heads 12, 13 are arranged on each side of the vertical path of the web, the two heads being horizontally spaced and equidistant from the web. Above the spray heads, the web in its vertical travel passes through a drying oven 14.

Each spray head includes an inclined blade 15, preferably inclined at about 45 to the horizontal, supported and stiffened by the brackets 16 and mounted on insulators 17. At its upper end the blade carries a distribution trough 18, having a horizontal edge 18a constituting a weir over which coating composition must pass to flow down the blade. A perforated distribution tube 19 carries composition from a. source of the composition (not shown) through the feed lines 20 and maintains the desired level of composition in the trough 18. The lower end of the blade terminates in a sharp edge 21 at which atomisation of the composition takes place.

A retainer blade 22 with an accurately worked lower edge 23 is carried by brackets 24 provided with screw adjustments. The lower edge 23 of the retainer blade is narrowly spaced from the upper surface of the blade 15, the spacing determining the volume of liquid composition flowing to the sharp edge 21 in unit time. It is preferred that the lower edge 23 of the retainer blade should be at a point more than half way down the inclined blade 15, but may be at any convenient distance from the sharp edge. The lateral extremities of the lower edge 21 carry brass spheres 25 which serve to control the field at the edges of the blade.

Electrical connections are made by means of the leads 26 between the blade of each spray head 12, 13 and a high tension D.C. generator, which is arranged to charge the blades to equal and opposite high potentials with respect to earth.

Instead of the feed trough and weir device, each spray head may comprise an inclined doctor blade with a feed roller in contact with the upper edge thereof, as also described in the said co-pending application.

Drying apparatus, such as the oven 14,, may be provided on that portion of the path of the web lying between the spraying zone and the take-up device.

In carrying out the method of the invention on this apparatus, the distance between the atomising edge 21 of the blade and the web 10 may be varied considerably, for example between 4 and 7 inches, the best value in a given case depending principally on the nature of the coating composition used, the drying characteristics being especially important. In most cases, a distance of 5 to 6 inches is preferred. The preferred potentials lie within the range of numerical magnitude of 55-80 kv. on each spray head. The maximum potential obtainable is dependent upon the spacing of the spray heads, and may be increased to a value just below that at which arcing occurs. In some circumstances, potentials as high as kv. may be used.

The following is an example of a coating composition which can be applied to paper according to the invention and dried at 120 C. to provide a polyvinyl chloride protective coating which imparts oil and grease resistance.

Parts Partially hydrolysed vinyl chloride/ vinyl acetate copolymer about 91% vinyl chloride (VAGH-Union tional step of causing said compositions to flow evenly Carbide Corporation) 3.64 Blltyl acetate 14.2 Methyl isobutyl ketone 9.65 Tetrahydrofurfuryl alcohol 1.0 Butyl alcohol 1.28 Polypropylene laurate 1.21

The following is an example of a coating composition which can be applied according to the invention to primed polypropylene, and dried at 110 C., to provide a polyvinylidene chloride protective coating, which will impart improved gas permeability and heat sealing properties:

What is claimed is:

1. A method of coating both sides of a web of electrically non-conductive sheet material which comprises the steps of:

passing said web between two spaced apart and mutually confronting electrostatic spray heads;

maintaining a high potential dilference between said spray heads with one head at a polarity opposite to that of the other head;

supplying fluid coating material to said two spray heads and causing said material to be atomised by the field established by said high potential difference simultaneously at both said heads;

and depositing the atomised coating material under the influence of said field simultaneously on both sides of said web from said spray devices on respective opposite sides thereof.

2. A method according to claim 1 in which the web is drawn in a generally vertical direction between and equidistant from horizontally spaced spray heads.

3. A method according to claim 1 in which spray heads on opposite sides of the web are maintained at potentials of substantially equal magnitude and opposite sign.

4. A method of coating both sides of a web of electrically non-conductive sheet material which comprises the steps of:

passing said web between two spaced apart and mutually confronting electrostatic spray heads;

supplying fluid coating material to said two spray heads;

maintaining a high potential difference between said spray heads with one head at a polarity opposite to that of the other head and causing said coating material to be atomised by the field resulting from said high potential difference simultaneously from said spray heads;

and depositing the atomised coating material under the influence of said field simultaneously on both sides of said web from said spray heads on respective opposite sides thereof.

5. A method according to claim 4 including the addidown inclined planes forming part of said respective spray heads to be atomised at sharp edges found at the lower edges of said planes.

6. Apparatus for coating both sides of a web of electrostatically non-conductive sheet material comprising;

two spray heads'in spaced apart and mutually confronting relationship said head being of the electrostatic atomising type; 1

supply means for supplying fluid coating material to said two spray heads;

web advancing means for drawing said webs between said spaced apart heads;

and electrostatic means for maintaining a high potential difference between said heads whereby coating material is atomised at both said heads and deposited simultaneously on both sides of said web.

7. Apparatus according to claim 6 in which the heads are spaced horizontally from one another and the web is drawn between them in a generally vertical plane substantially equidistant from the two heads.

8. Apparatus according to claim 6 in which each spray head comprises an inclined plane of which the free lower edge is formed as a sharp edge for electrostatic atomisation and means for supplying the composition evenly to the upper end of the inclined plane.

9. Apparatus for coating both sides of a web of electrically non-conducting sheet material, said apparatus comprising: two horizontally spaced, inwardly inclined, mutually opposed, inclined planes each having a free lower edge, said edges being formed as sharp edges for electrostatic atomisation; liquid supply means at the upper end of said planes to supply liquid coating composition evenly to said planes; web advancing means for drawing a web between said opposed sharp edges and means for maintaining a high potential difference between said edges sufiicient to cause atomisation of said composition.

10. Apparatus according to claim 9, including a distribution trough having a horizontal edge over which the composition can flow onto the upper end of the plane, and a perforated distribution tube arranged to feed the composition to the trough.

11. Apparatus according to claim 9 including a retainer blade closely spaced from the surface of the inclined plane and beneath which the composition must flow to reach the lower edge of the plane.

References Cited UNITED STATES PATENTS 3,147,137 9/1964 Glass et al 117-68 596,905 1/ 1898 McCoy 118-620 1,686,426 10/1928 Waldron et al 118---315 2,048,651 7/1936 Norton 11793.4 2,287,837 6/1942 Smyser 118638 X 2,651,287 9/1953 Turner 118-636 X 2,748,018 5/ 1956 Miller 117--17 2,809,128 10/1957 Miller 239-3 X 2,911,323 11/1959 Prance 2393 X 2,993,808 7/ 1961 Thornberry 118-638 X 3,024,132 3/ 1962 Mathues 117-93.4

FOREIGN PATENTS 853,722 2/ 1962 France.

' ALFRED L. LEAVITT, Primary Examiner.

MURRAY KATZ, Examiner.

A. GOL'IAN, Assistant Examiner. 

1. A METHOD OF COATING BOTH SIDES OF A WEB OF ELECTRICALLY NON-CONDUCTIVE SHEET MATERIAL WHICH COMPRISES THE STEPS OF: PASSING SAID WEB BETWEEN TWO SPACED APART AND MUTUALLY CONFRONTING ELECTROSTATIC SPARY HEADS; MAINTAINING A HIGH POTENTIAL DIFFERENCE BETWEEN SAID SPRAY HEADS WITH ONE HEAD AT A POLARITY OPPOSITE TO THAT OF THE OTHER HEAD; SUPPLYING FLUID COATING MATERIAL TO SAID TWO SPRAY HEADS AND CAUSING SAID MATERIAL TO BE ATOMISED BY THE FIELD ESTABLISHED BY SAID HIGH POTENTIAL DIFFERENCE SIMULTANEOUSLY AT BOTH SAID HEADS; AND DEPOSITING THE ATOMISED COATING MATERIAL UNDER THE INFLUENCE OF SAID FIELD SIMULTANEOUSLY ON BOTH SIDES OF SAID WEB FROM SAID SPRAY DEVICES ON RESPECTIVE OPPOSITE SIDES THEREOF. 